Accepted Manuscript Title: Synthesis and flocculation properties of gum ghatti and poly(acrylamide-co-acrylonitrile) based biodegradable hydrogels Author: Hemant Mittal Rajeev Jindal Balbir Singh Kaith Arjun Maity Suprakas Sinha Ray PII: DOI: Reference:
S0144-8617(14)00798-X http://dx.doi.org/doi:10.1016/j.carbpol.2014.08.029 CARP 9166
To appear in: Received date: Revised date: Accepted date:
13-6-2014 30-7-2014 7-8-2014
Please cite this article as: Mittal, H., Jindal, R., Kaith, B. S., Maity, A., and Ray, S. S.,Synthesis and flocculation properties of gum ghatti and poly(acrylamideco-acrylonitrile) based biodegradable hydrogels, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.08.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Submitted to Carbohydrate Polymers
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Revised version
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Synthesis and flocculation properties of gum ghatti and poly(acrylamide-
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co-acrylonitrile) based biodegradable hydrogels
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Hemant Mittala,b *, Rajeev Jindalc, Balbir Singh Kaithc, Arjun Maityb and Suprakas Sinha
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Raya, b *
Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, South Africa b
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Department of Chemistry, National Institute of Technology Jalandhar 14401, Punjab, India
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DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa
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*Corresponding authors: Fax: +27128412229; E-mails: [email protected] (HM)
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and [email protected] (SSR)
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Abstract
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This article reports the development of biodegradable flocculants based on graft co-polymers
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of gum ghatti (Gg) and a mixture of acrylamide and acrylonitrile co-monomers (AAm-co-
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AN). The hydrogel polymer exhibited an excellent swelling capacity of 921% in neutral
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medium at 60°C. The polymer was used to remove saline water from various petroleum
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fraction-saline water emulsions. The flocculation characteristics of the hydrogel polymer
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were studied in turbid kaolin solution as a function of the amount of polymer and the solution
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temperature and pH. Biodegradation studies of hydrogel polymer were conducted using the
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soil composting method, and the degradation process was constantly monitored using
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scanning electron microscopy and Fourier transform infrared spectroscopy techniques. The
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results demonstrated an 89.47% degradation of the polymer after 60 days. Finally, the
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hydrogel polymer adsorbed 98% of cationic dyes from the aqueous solutions.
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Keywords: gum ghatti; hydrogel; swelling; flocculation; biodegradation; adsorption
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1.
Introduction
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In recent decades, synthetic polymers have been extensively used by many industries because
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of their excellent mechanical and physical properties. However, the main disadvantage of
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synthetic polymers is their non-degradable nature, which makes their disposal difficult.
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Recently, researchers have made a variety of attempts to replace non-degradable polymers
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with biodegradable polymers (Averous, & Pollet, 2013; Hermanova, et al., 2013; Okada et
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al., 2002; Liang, et al., 2013; Zare, Lakouraj, & Mohseni, 2014). Biodegradation is the
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process of the fragmentation of long polymer chains into low molecular weight compounds in
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the presence of microorganisms. Biodegradation of polymers depends upon several factors
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including the chemical and physical properties of polymers, the composition of the final
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product and the length and crystallinity of the polymer chains (Abrusci, Marquina, &
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Catalina, 2007; Nageotte, Pester, Pradet, & Bayard, 2006). The experimental conditions
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including temperature and humidity also influence the biodegradability of polymers.
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While biopolymers are highly biodegradable, they tend to have poor properties compared to
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synthetic polymers. Biopolymers also have some potential advantages over synthetic
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polymers, including their excellent biocompatibility, their eco-friendly nature, low cost,
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abundant availability, and suitability for use in many biomedical applications (Jenkins,
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Samuel, & Hudson, 2001; Park, Park, & Kim, 2006). Moreover, the inherent properties of
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biopolymers can be improved through a variety of techniques including graft co-
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polymerization. Graft co-polymerization is the most widely studied technique to improve the
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inherent properties of biopolymers, and it has mainly been used for polysaccharide-based
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biopolymers. Recently, many studies have been carried out on the graft co-polymerization of
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gum polysaccharides such as gum ghatti, gum xanthan, gum arabic and guar gum (Kaith,
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Ranjta, & Kumar, 2008; Mittal, Fosso-Kankeu, Mishra, & Mishra, 2013; Mittal, Ballav, &
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Mishra, 2014; Sand, Yadav, Mishra, Tripathi, & Behari, 2011; Sanghi, Bhattacharya, &
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Singh, 2006), and the resulting graft co-polymers have potential applications in the food
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industry, as drug delivery devices, in soft tissue engineering and waste water treatment
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(Mariano, El Kissi, & Dufresne, 2014; Mittal, & Mishra, 2014; Sand, Yadav, & Behari, 2010;
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Sarkar, Mandre, Panda, & Pal, 2013; Thakur, Thakur, Raghavan, & Kessler, 2014).
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The anionic polysaccharide gum ghatti (Gg) is an exudate of the Anogeissus latifolia tree,
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which belongs to the Combretaceae family. The structure of Gg is composed of sugars such
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as L-arabinose, D- galactose, D-mannose, D-xylose and D-glucoronic acid in a 48:29:10:5:10
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molar ratio (Aspinall, Hirst, & Wickstrom, 1955). The molecular structure of Gg contains
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alternating 4-O-substituted and 2-O-substituted α-D-mannopyranose units and chains of 1Æ6
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linked β-D-galactopyranose units with side chains that are most frequently single L-
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arabinofuranose residues (Aspinall, 1980; Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia,
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2103) (Fig. S1, supplementary data).
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The process of removing different organic and inorganic impurities from waste water by
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converting suspended particles into heavy flocs is called flocculation. Flocculation is a highly
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efficient technique for the purification of industrial and domestic wastewater and the
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thickening of sludge. Flocculants can be either inorganic, usually called coagulant agents
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(such as aluminum sulfate and ferric chloride), or polymeric materials (Ghimici, Constantin,
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& Fundueanu, 2010). The properties of polymer-based flocculants can be easily tailored to
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specific applications. Moreover, the flocculation capacity of polymer-based flocculants is
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much higher than that of inorganic flocculants (Singh et al., 2000). Polymer-based flocculants
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can be synthetic (Ghimici, Dranca, Dragan, Lupascu, & Maftuleac, 2001; Nasser, & James,
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2006) or natural (Ghimici, & Constantin, 2011; Pal, Sen, Karmakar, Mal, & Singh, 2005;
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Simon, Picton, Le Cerf, & Muller, 2005). Synthetic polymer-based flocculants exhibit better
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performance than natural polymer-based flocculants. However, natural polymer-based
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flocculants have their own advantages including low cost, biodegradability and non-toxicity
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(Ghimici, Constantin, & Fundueanu, 2010). Therefore, it is critical to develop flocculant
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materials with the advantages of both synthetic and natural polymers. Recently, a great deal
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of work has been reported aiming to develop flocculants through the graft co-polymerization
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of polysaccharides with synthetic vinyl monomers, such as poly(acrylic acid) and
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poly(acrylamide) (Chang, Hao, & Duan, 2008; Mittal, Mishra, Mishra, Kaith, Jindal, &
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Kalia, 2103; Singh et al., 2000; Tian, & Xie, 2008; Wan, Li, Wang, Chen, & Gu, 2007).
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The purification of industrial effluent water is one of the greatest challenges faced by
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developing countries. Industries producing batteries, textiles, paper, cosmetics and ink
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generate huge amounts of wastewater contaminated with various types of dyes. It is very
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difficult to degrade dyes in sludge treatment plants because most dyes are non-degradable in
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nature (Kumari, & Abraham, 2007). Adsorption is one of the most efficient techniques used
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for the treatment of dye-contaminated water. In this direction, gum polysaccharide-based
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graft co-polymers have demonstrated their potential to efficiently adsorb dyes from
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wastewater (Ghorai, Sarkar, Raoufi, Panda, & Schonherr, 2014; Mittal, & Mishra, 2014;
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Mittal, Ballav, & Mishra, 2014).
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While researchers have tested these polymers for their flocculation characteristics, swelling
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properties and ability to adsorb pollutants from contaminated water, these polymers do not
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exhibit these key properties, resulting in poor performance. Furthermore, synthesized graft
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co-polymers exhibited considerable resistance to biodegradation (Mishra, Sand, Mishra,
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Yadav, & Behari, 2010; Sand, Yadav, & Behari, 2010; Sand, Yadav, Mishra, Tripathy, &
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Behari, 2011). In previous studies, we also reported the swelling properties, flocculation
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characteristics and biodegradation of cross-linked graft co-polymers of Gg with AAm (Mittal,
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Mishra, Mishra, Kaith, & Jindal, 2103) and with the co-polymer mixture of AAm with the
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hydrophilic vinyl monomer acrylic acid (AA) (Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia,
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2103). The current study reports the effect of a co-polymer mixture of AAm with a
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hydrophobic vinyl monomer, i.e., acrylonitrile (AN) on the flocculation characteristics,
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swelling properties, thermal stability, adsorption efficiency and biodegradation behavior of a
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cross-linked graft co-polymer of Gg.
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2.
Experimental
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2.1.
Materials
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AAm, AN, Gg, N, N’-methylene-bis-acrylamide (MBA), potassium persulfate (KPS),
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ascorbic acid (ABC), petroleum ether, methylene blue, rhodamine B and kaolin were
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purchased from Merck Chemicals (India) and used as received. Petrol and diesel were
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purchased from Indian Oil Corporation Limited, India. All chemicals were of analytical grade
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and were used as received without further purification. All solutions were prepared in triple-
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distilled water. Flocculation studies were conducted on a Micro 1000 IR Turbidimeter (HF
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Scientific Incorporation, USA). Stock solutions of dyes (1000 mg.l-1) were prepared by
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dissolving the appropriate amount of dye in sterile distilled water; adequate volumes of the
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stock solutions were further diluted in distilled water to prepare experimental solutions.
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2.2.
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The cross-linked hydrogel polymer of Gg with the co-polymer mixture of P(AAm-co-AN)
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was prepared using graft co-polymerization (Jindal, Kaith, & Mittal, 2010). At first, 1 g of Gg
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was added to 30 ml distilled water and left undisturbed for 24 h to obtain a homogeneous
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mixture of gum polysaccharide in water. After 24 h, a mixture of KPS (30 mg) and ABC (20
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mg) was added to the reaction mixture with constant stirring, followed by the addition of
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MBA (50 mg). The reaction temperature was maintained at 60 °C. The monomer mixture of
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AAm (1.0 g) and AN (2.0 ml) was added to the reaction vessel. The reaction was allowed to
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continue for the next 6 h. After that, the reaction vessel was removed and cooled to room
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temperature. The homopolymers polyacrylamide and polyacrylonitrile and the unreacted co-
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polymer P(AAm-co-AN) were removed by successive extractions with acetone and N,N’-
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dimethylformamide, respectively. Finally, the synthesized hydrogel was dried in a hot air
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oven at 60 °C until a constant weight was achieved.
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Synthesis of the Gg-cl-P(AAm-co-AN) hydrogel polymer
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2.2.1. Characterization
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Changes in the thermal behavior of Gg after graft co-polymerization and cross-linking with
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P(AAm-co-AN) and MBA were studied using TGA/DTA/DTG (TG/DTA 6300, SII
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EXSTAR 6000, SII Nanotechnology Incorporation, Japan) in air at a heating rate of 10 °C
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per minute. The FTIR spectra of the Gg and the Gg-cl-P(AAm-co-AN) hydrogel polymer
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before degradation and at various stages of degradation were recorded on a PERKIN ELMER
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RXI spectrophotometer using KBr pellets (Sigma Aldrich) in the spectral range of 4000-400
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cm-1 with a resolution of 4 cm-1. The morphological change of the hydrogel polymers at
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various stages of biodegradation were examined using SEM (LEO, 435 VF, LEO Electron
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Microscopy Ltd., Japan). As all samples were non-conductive, they were coated with gold to
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introduce conductivity.
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2.3.
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As the ability to swell in water is one of the most important features of hydrogels, it is very
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important to determine the swelling capacity of the synthesized hydrogel polymer. The
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swelling efficiency of the Gg-cl-P(AAm-AN) hydrogel polymer was investigated at various
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time intervals (4, 8, 12, 16, 20 and 24 h), temperatures (30, 40, 50, 60 and 70 °C) and pH
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values (3.0 to 13.0) in deionized water. For each experiment, several 100 mg of samples of
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hydrogel polymer were each immersed in 100 ml of deionized water. After a particular time
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interval, the samples were removed, gently wiped and weighed. The percentage swelling (Ps)
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was then calculated using Eq. (1) (Kaith, Jindal, Mittal, & Kumar, 2012):
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Swelling studies in deionized water
Ps = 166
W s − Wd × 100 Wd
(1)
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where Ws and Wd are the weights of the swelled and dry hydrogel polymer samples,
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respectively.
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After the swelling time was optimized, the effect of temperature on Ps was studied at a pre-
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optimized time, and then the pH of the swelling medium was optimized to obtain the
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maximum Ps at the pre-optimized time and temperature.
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2.3.1.
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Selective absorption of saline from various petroleum fraction-saline
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emulsions
The separation of saline water from crude petroleum during their extraction from the sea is
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challenging because current processes are highly tedious and expensive. The hydrogel
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polymers synthesized here were tested for the selective absorption of saline water from
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various petroleum fraction-saline emulsions. A 1% NaCl solution was used as saline water
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with various petroleum fractions such as kerosene, diesel, petrol and petroleum ether. From
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each emulsion, 50 ml (i.e., 25 ml of saline water and 25 ml of the petroleum fraction) was
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collected in a 250 ml conical flask. A stable emulsion was prepared by adding 10 ml tween-
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20, and the flasks were shaken using an automatic shaker machine (LABCO, India). Once a
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stable emulsion was obtained, 100 mg of hydrogel polymer was added. After 16 h, the flasks
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were removed from the shaker and allowed to stand undisturbed for 48 h to break up the
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emulsions. The petroleum fractions were then separated using a separating funnel, and the
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remaining unabsorbed volume of saline water was measured.
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2.4.
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Flocculation characteristics of the Gg-cl-P(AAm-AN) hydrogel polymer were investigated
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using the standard jar test (Jha, Agarwal, Mishra, & Rai, 2001; Mittal, Mishra, Mishra, Kaith,
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& Jindal, 2013). For each experiment, a volume of 500 ml of the 50 ppm kaolin solution was
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placed in 1000 ml and agitated at 180 rpm for 10 min. Initially, a blank reading was taken
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followed by the addition of hydrogel polymer to the kaolin suspension. Readings were taken
Flocculation studies
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every 10 min until almost constant turbidity was obtained. Different doses of the hydrogel
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polymer were used to determine the optimal dose to maximize flocculation capacity. The
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flocculation temperature was then optimized, followed by the solution pH.
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2.5.
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The biodegradation of the Gg-cl-P(AAm-AN) hydrogel polymer was studied using the soil
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composting method (Kaith, Jindal, Maiti, & Jana, 2010). Biodegradation studies were
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performed in flower pots, and the compost was collected from the waste water effluent
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discharge plant of the National Institute of Technology, Jalandhar, India (Fig. 1). For each
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experiment, rectangular samples (2 x 2 x 1.5 cm) were prepared and buried in soil compost in
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a flower pot at a depth of 2 cm, 3 cm apart from each other. Microbe rich water from the
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plant was added to the compost on alternating days to compensate for water loss caused by
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evaporation. The progress of the degradation process was examined every 5 days. Every 5
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days, the samples were extracted from the compost, carefully cleaned, dried at 50 °C in a
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vacuum oven and weighed.
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The percentage weight loss at each stage of degradation was calculated using Eq. (2) (Kaur,
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Bhalla, Deepika, & Gautam, 2009):
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Biodegradation studies
Weight loss (%)
=
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where Wi is the initial weight of the hydrogel polymer and Wt is the weight after a particular
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time interval.
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The progress of degradation was also evaluated by studying the changes in physical
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appearance and chemical composition using SEM and FTIR, respectively.
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2.6.
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Adsorption studies of the cationic dyes methylene blue and rhodamine B were carried out in
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batch mode. A 50 mg·l-1 solution of each dye (50 ml) was first placed in 100 ml glass bottles.
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Various amounts of the Gg-cl-P(AAm-co-AN) hydrogel polymer were added to each dye
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solution. The bottles were placed in a thermostatic water bath and shaken for 24 h at 200 rpm.
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After 24 h, the bottles were removed from the water bath and the dye solutions were filtered
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using a 0.45 µm cellulose acetate syringe filter. The concentration of unadsorbed dye
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remaining in the solution was analyzed using a UV/vis spectrophotometer (Shimadzu, Japan
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UV-2450, UV-vis spectrophotometer) at 554 nm as the λmax of rhodamine B and 664 nm as
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the λmax of methylene blue.
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calculated using Eq. (3) (Sadeghi, Azhdari, Arabi, & Moghaddam, 2012):
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The adsorption efficiency or the percentage removal was
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3.1.
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Adsorption of methylene blue and rhodamine B from the aqueous solution
Results and Discussion
Thermogravimetric analysis, differential thermal analysis and differential thermo-gravimetric analysis (TGA/DTA/DTG)
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The initial decomposition temperature (IDT) of the Gg-cl-P(AAm-co-AN) hydrogel polymer
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was found to be 133.6 °C (Fig. S2, Supplementary Data), which was much lower than the
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IDT of the Gg, i.e., 206.9 °C (Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). In contrast, the
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final decomposition temperature (FDT) of the Gg-cl-P(AAm-co-AN) hydrogel polymer
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(681.8 °C) was found to be much higher than the FDT of the Gg (521.6 °C) (Mittal, Mishra,
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Mishra, Kaith, & Jindal, 2013). The graft co-polymerization of the Gg with the binary
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monomer mixture of P(AAm-co-AN) decreases the IDT. This may be associated with the
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degradation of P(AAm-co-AN) chains as a result of the formation of the imide group with the
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evolution of ammonia. Furthermore, as the formation of amide groups increased the FDT of
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the graft co-polymer, a higher FDT was observed in the case of the Gg-cl-P(AAm-co-AN)
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hydrogel polymer compared to Gg (Behari, Pandey, Kumar, & Taunk, 2001). The Gg-cl-
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P(AAm-co-AN) hydrogel polymer degradation curve also exhibited three stages of
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decomposition ranging from 133.6-276.8 °C (weight loss, 17.3%), 276.8-436.2 °C (weight
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loss, 18.3%) and 436.2-681.1 °C (weight loss, 61.3%). In contrast, the TGA of the Gg
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exhibited two stage decomposition over the temperature ranges from 206.9-331.4 °C (weight
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loss, 46.6%) and 331.4-521.6 °C (weight loss, 31.2%). Moreover, a detailed study of the
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percentage weight loss at different temperatures indicated that in Gg, a certain percentage
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weight loss occurred at lower temperatures compared to the Gg-cl-P(AAm-co-AN) hydrogel
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polymer (Table S1, Supplementary Data).
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The DTA of the Gg exhibited two exothermic peaks at 483.2 °C (203 µV) and 492.7 °C (154
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µV), whereas the DTA of the Gg-cl-P(AAm-co-AN) hydrogel polymer exhibited exothermic
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peaks at 280.8 °C (102.2 μV) and 669.9 °C (136.2 μV). DTA peaks were observed at much
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higher temperatures with less heat evolved in the DTA of the Gg-cl-P(AAm-co-AN) (Fig. S2,
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Supplementary Data) compared to the Gg (Mittal, Mishra, Mishra, Kaith, & Jindal, 2013).
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The Gg-cl-P(AAm-co-AN) hydrogel polymer exhibited two DTG peaks at 280.8 °C with a
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mass loss rate of 2.223 mg/min and at 669.4 °C with a mass loss rate of 1.01 mg/min (Fig.
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S2, Supplementary Data). In contrast, the DTG of the Gg exhibited decompositions at 299.1
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°C and 479.0 °C with mass loss rates of 1.463 and 2.23 mg/min, respectively (Mittal, Mishra,
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Mishra, Kaith, & Jindal, 2013). DTG studies showed that the rate of thermal decomposition
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was higher in Gg than in the Gg-cl-P(AAm-co-AN) hydrogel polymer. Thus,
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TGA/DTA/DTG studies suggest that the thermal stability of Gg improved after including
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P(AAm-co-AN) chains in the backbone polymer through graft co-polymerization and cross-
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linking.
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3.2.
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Swelling studies of the Gg-cl-P(AAm-co-AN) hydrogel polymer in deionized water
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3.2.1.
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The effect of swelling time on Ps was studied at various time intervals (4, 8, 12, 16, 20 and 24
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h) (Fig. 2(a)). Initially, the Ps was found to increase with increasing swelling time, reaching a
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maximum Ps (669%) after 16 h. Further increases in swelling time did not lead to any further
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increases in swelling capacity, and almost a constant value of Ps was observed. This may be
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because after reaching maximum swelling, the three dimensional porous network of the Gg-
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cl-P(AAm-co-AN) hydrogel polymer becomes fully saturated, with no more active sites
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available to accommodate water molecules. Therefore, no increase in swelling capacity was
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observed after 16 h swelling time.
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Effect of time on Ps
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3.2.2.
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The effect of temperature on Ps was studied at various temperatures (30, 40, 50, 60 and 70
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°C) at the pre-optimized swelling time (16 h) (Fig. 2(b)). Initially, swelling capacity was
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found to increase with increasing temperature up to 60 °C, reaching a maximum value of Ps
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(921%). However, with further increases in temperature, the Ps of the Gg-cl-P(AAm-co-AN)
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hydrogel polymer decreased remarkably, reaching 863% at 70 °C. This suggests that
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increases in temperature initially increased the elasticity of the cross-linked hydrogel polymer
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network, resulting in a larger Ps, but above the optimal temperature the polymer network
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collapsed, leading to desorption with further increases in temperature (Zhang, Yang, Chung,
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& Ma, 2001). Moreover, the hydrophilic groups of the hydrogels formed hydrogen bonds
Effect of temperature on Ps
Page 12 of 37
with water molecules. These bonds cooperatively formed a stable shell of hydration around
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the hydrophilic groups, increasing the uptake of water and thus increasing Ps. However, the
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associated interactions among the hydrophilic groups released the entrapped water molecules
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from the hydrogel network at higher temperatures. Therefore, a lower Ps value was observed
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at higher temperatures (Feil, Bae, Feijen, & Kim, 1992; Inomato, Goto, & Saito, 1990).
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3.2.3.
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Solution pH was found to have a profound effect on the swelling of hydrogel polymers, as
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reported by a number of researchers (Mahdavinia, Pourjavadi, Hosseizadeh & Zohurian,
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2004; Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). The effect of pH on the Ps of the Gg-cl-
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P(AAm-co-AN) hydrogel polymer was studied in solutions with pH values ranging from 3.0
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to 13.0 at the pre-optimized swelling time and temperature (Fig. 2(c)). A higher value of Ps
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was observed in the solution at neutral pH (921%) compared to acidic and alkaline pH
304
solutions. The swelling capacity of the hydrogel polymer increased with solution pH. This
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type of swelling behavior can be explained based on osmotic swelling pressure theory. In the
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case of dilute electrolyte solutions, the swelling capacity of the hydrogel polymers depends
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on osmotic swelling pressure (πion), given as (Bajpai, 1999):
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Effect of pH on Ps
πion = RT∑ (Cig - Cis)
(4)
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where Cig and Cis are the molar concentrations of mobile ions in the swollen gel and external
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solution, respectively. R and T are the gas constant and absolute temperature, respectively.
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In the solutions of neutral medium, the concentration of mobile ions (Cis) in swelling medium
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became very small, increasing the value of πions. In contrast, in acidic solutions, most of the
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carboxylate ions were protonated (Mahdavinia, Pourjavadi, Hosseizadeh & Zohurian, 2004),
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decreasing the concentration of mobile ions within the swollen gel (Cig), leading to a reduced
Page 13 of 37
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value of πions. Furthermore, in alkaline solutions, –COOH groups completely dissociated and
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Na+ and –OH ions were present at very high concentration, reducing the values of πions and Ps
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(Mahdavinia, Pourjavadi, Hosseizadeh & Zohurian, 2004; Mittal, Mishra, Mishra, Kaith, &
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Jindal, 2013).
3.2.4.
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Selective absorption of saline from different petroleum fraction-saline emulsions
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Initially, the swelling behavior of the Gg-cl-P(AAm-co-AN) hydrogel polymer was tested in
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different petroleum fractions, i.e., petrol, kerosene, diesel and petroleum ether, and the
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hydrogel polymer did not absorb any of these petroleum fractions. The maximum saline
325
adsorption was observed in the petroleum ether-saline emulsion (25.5%) followed by the
326
petrol-saline emulsion (23.5%), the kerosene-saline emulsion (21.0%) and the diesel-saline
327
emulsion (18.7%) (Table 1). This adsorption trend can be attributed to the length of
328
hydrocarbon chains present in the various petroleum fractions. Petroleum fractions with
329
larger hydrocarbon chains have large hydration shells, impeding the absorption of water
330
molecules by the hydrogel polymer. As petroleum ether has the fewest carbon atoms (C15), the maximum adsorption
332
of saline was observed in the petroleum ether-saline emulsion, followed by the petrol-saline
333
emulsion, the kerosene-saline emulsion and the diesel-saline emulsion (Mittal, Mishra,
334
Mishra, Kaith, Jindal, & Kalia, 2013).
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3.3.
Characterization of the flocculation of the Gg-cl-P(AAm-co-AN) hydrogel polymer
3.3.1. Effect of polymer dose on flocculation efficiency
Page 14 of 37
The effect of the amount of the Gg-cl-P(AAm-co-AN) hydrogel polymer on the turbidity of
340
the kaolin suspension was studied in the presence of various masses of hydrogel polymer (10,
341
15, 20 and 25 mg.l-1) in a neutral medium. Initially, the turbidity of the kaolin suspension
342
decreased with increasing amounts of hydrogel polymer, reaching a minimum value of 2144
343
NTU with 15 mg·l-1 polymer (Fig. 3(a)). However, further increases in the amount of
344
hydrogel polymer resulted in increases in turbidity. The gradual initial decrease in the
345
turbidity of the kaolin suspension was due to the formation of particle-polymer-particle
346
complexes between the kaolin particles and the Gg-cl-P(AAm-co-AN) hydrogel polymer,
347
where the hydrogel polymer acted as a bridge between different suspended kaolin particles
348
(Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia, 2013; Tian, & Xie, 2008). Kaolin particles
349
attached to different functional groups present on the polymer chain as soon as they came
350
into contact with the hydrogel polymer, leading to the formation of extended polymer chains.
351
In the presence of lower amounts of polymer, insufficient functional groups were available to
352
act as bridges between different clay particles. However, as the polymer dose was increased
353
the number of functionalities increased, increasing the number of bridges accessible for the
354
formation of particle-polymer-particle complexes, decreasing the turbidity value. Therefore,
355
increasing the amount of hydrogel polymer initially resulted in a gradual decrease in
356
turbidity. Further increases in the amount of hydrogel polymer beyond the optimum increased
357
the turbidity of the kaolin suspension because beyond the optimum dose, kaolin particles will
358
envelope between different polymer chains, thus failing to come into contact with the
359
functional groups of the hydrogel polymer (Xie, Feng, & Cao, Xia, & Lu, 2007). Kaolin
360
particles in suspension will be re-stabilized, resulting in a higher turbidity value. Therefore,
361
particle-polymer-particle complexes will not be formed in the presence of excessive amounts
362
of hydrogel polymer, resulting in a higher turbidity value (Ghimici, & Nichifor, 2010; 2012;
363
Ghimici, Constantin, & Fundueanu, 2012).
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Page 15 of 37
3.3.2. Effect of temperature on flocculation efficiency
365
The effect of temperature on the flocculation efficiency of the Gg-cl-P(AAm-co-AN)
366
hydrogel polymer is shown in Fig. 3(b). Initially, turbidity was found to decrease with
367
increasing temperature, reaching a minimum value of 1915 NTU at 40 °C. Further increases
368
in temperature beyond the optimum value increased the turbidity of the kaolin solution. The
369
initial decrease in the turbidity of the kaolin solution was associated with the formation of
370
particle-polymer-particle complexes and was supported by the initial increase in temperature.
371
However, at higher temperatures, the kinetic energy of hydrogel polymer molecules was
372
increased, resulting in the re-dispersion of kaolin particles and a higher turbidity value
373
(Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia, 2013).
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M
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3.3.3. Effect of pH on flocculation efficiency
376
The effect of the solution pH on the flocculation efficiency of the Gg-cl-P(AAm-co-AN)
377
hydrogel polymer was examined in kaolin solutions of various pH values at the pre-optimized
378
polymer dose and temperature (Fig. 3(c)). The minimum turbidity of the kaolin solution was
379
observed at acidic pH, and the turbidity of the kaolin solution increased with increasing
380
solution pH, with the maximum turbidity observed in the kaolin solution at alkaline pH. The
381
turbidity was lower in acidic solutions because under acidic conditions, the hydroxyl and
382
carboxylate groups present in the Gg-cl-P(AAm-co-AN) hydrogel polymer were protonated,
383
imparting a cationic charge to the overall hydrogel polymer network (Mittal, Mishra, Mishra,
384
Kaith, Jindal, & Kalia, 2013). Negatively charged kaolin particles were easily attached to
385
those cationic sites, forming larger flocks that easily settled, resulting in a reduction in the
386
turbidity of the kaolin solution. In alkaline solutions, amides are weakly acidic and resonance
387
stabilized (Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). Therefore, due to the anionic-
388
anionic repulsion between the functional groups of the Gg-cl-P(AAm-co-AN) hydrogel
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389
polymer and kaolin particles, particle-polymer-particle complexes were not formed and little
390
decrease in the turbidity of the kaolin solution was observed.
391 392
3.4.
393
While biopolymers are well-suited for many applications due their biodegradable nature, they
394
also have serious shortcomings for many applications. Therefore, many biopolymers need to
395
be chemically modified without affecting the biodegradable nature of the biopolymer. Here,
396
we studied the effect of graft co-polymerization and crosslinking of Gg with the binary
397
monomer mixture of P(AAm-co-AN) on its biodegradability. In the Gg-cl-P(AAm-co-AN)
398
hydrogel polymer, the process of biodegradation starts with the backbone polymer, i.e., Gg.
399
The weight of the hydrogel polymer was measured every 5 days (Table 2). The
400
biodegradation process was found to be continuous in terms of the percentage weight loss of
401
the Gg-cl-P(AAm-co-AN) hydrogel polymer versus time. The Gg-cl-poly(AAm-co-AA)
402
hydrogel polymer degraded by up to 89.47% over 60 days. The polymer samples were both
403
enzymatically and chemically degraded. Furthermore, the secretion products of the microbes
404
present in the composting media were also involved in the degradation of the hydrogel
405
polymer, leading to the cleavage of chemical bonds and bacterial degradation (Kaith, Jindal,
406
Maiti, & Jana, 2010).
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Biodegradation studies of the Gg-cl-P(AAm-co-AN) hydrogel polymer
408
3.4.1. Evidence of bio-degradation
409
3.4.1.1.
410
The FTIR spectrum of the Gg-cl-P(AAm-co-AN) hydrogel polymer was recorded at different
411
stages of biodegradation (Figs. 4(a-d)). The FTIR of the Gg-cl-P(AAm-co-AN) hydrogel
412
polymer before biodegradation exhibited peaks at 1673.25 cm-1 (C=O stretching of amide I
413
band), 1453.57 cm-1 (NH in-plane bending of the amide II band),
Spectroscopic studies
1253.44 cm-1 (CN
Page 17 of 37
stretching vibrations of the amide III band), 749 cm-1 (OCN deformations of the amide IV
415
band) and 2240.78 cm-1 (C≡N of acrylonitrile) (Fig. 4(a)) in addition to the characteristic
416
peaks of Gg (Kaith, Jindal, Mittal, & Kumar, 2012). However, the FTIR spectra of degraded
417
samples at different stages of biodegradation showed peaks with reduced intensity and
418
different peak positions compared to the FTIR spectrum of the Gg-cl-P(AAm-co-AN)
419
hydrogel polymer before degradation. Peaks initially appearing at 1673.25 cm-1, 2240.78 and
420
1453.57 cm-1 were shifted, with reduced intensity at biodegradation stage I (Fig. 4(b)). This
421
may be due to the initiation of the breakdown of the cross-linked network. However, the
422
degradation of individual compounds such as Gg and P(AAm-co-AN) began in the second
423
stage of biodegradation (Fig. 4(c)). The major process in this stage was the degradation of
424
Gg, together with the shifting or complete disappearance of the peaks related to the Gg. Most
425
of the peaks initially appearing in the FTIR spectrum of the Gg-cl-P(AAm-co-AN) hydrogel
426
polymer were completely shifted or absent in the third and final stage of biodegradation (Fig.
427
4(d)). Thus, FTIR studies at different stages of biodegradation confirmed that the grafted
428
chains of P(AAm-co-AN) and Gg were biodegraded in the soil compost.
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3.4.1.2.
431
The changes in the morphology of the Gg-cl-P(AAm-co-AN) hydrogel polymer at different
432
stages of biodegradation were monitored using SEM (Figs. 5(a-d)). Before the degradation
433
process started, the surface of the Gg-cl-P(AAm-co-AN) hydrogel polymer was found to be
434
smooth and homogeneous, with a continuous and regular morphology (Fig. 5(a)). However,
435
after the biodegradation process started, i.e., at the early stages of biodegradation
436
(biodegradation stage-I), the number of cracks on the surface of the hydrogel polymer
437
increased, and the homogeneous surface started becoming a fibrillar and heterogeneous
438
surface (Figs. 5(b-c)). Some fractures and fissures also started to appear on the surface of the
Morphology studies
Page 18 of 37
hydrogel polymer at the early stages of biodegradation, which confirmed the initiation of the
440
degradation process. These fractures and fissures on the surface of the hydrogel polymer may
441
be due to the initiation of the breakage of cross-links between different polymer chains,
442
beginning the process of degrading individual components of the hydrogel polymer (Mittal,
443
Mishra, Mishra, Kaith, Jindal, & Kalia, 2013). In the third and final stage of biodegradation,
444
the size and number of these fissures and fractures were drastically increased, and the
445
polymer surface became completely rough and heterogeneous due to the complete fractured
446
surface and the complete degradation of the individual components of the Gg-cl-P(AAm-co-
447
AN) hydrogel polymer (Fig. 5(d). This may be due to the complete breakage of covalent
448
bonds between different polymeric chains through chemical and enzymatic degradation by
449
the secreted products of different micro-organisms (Mittal, Mishra, Mishra, Kaith, Jindal, &
450
Kalia, 2013; Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). Thus, SEM studies predicted that
451
enzymes secreted by micro-organisms were responsible for the degradation of the Gg-cl-
452
P(AAm-co-AN) hydrogel polymer.
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454
3.5.
455
The Gg-cl-P(AAm-co-AN) hydrogel polymer was successfully used for the adsorption of
456
methylene blue and rhodamine B from aqueous solutions, and the results are shown in Fig. 6.
457
The percentage adsorption was found to increase as the amount of hydrogel polymer
458
increased. Almost 98% of methylene blue and rhodamine B were found to adsorb to 0.4 g·l-1
459
and 0.8 g·l-1 of the hydrogel polymer, respectively. We believe that the driving force for the
460
high adsorption of cationic dyes using the Gg-cl-P(AAm-co-AN) hydrogel polymer is the
461
electrostatic interaction between negatively charged functional groups of the hydrogel
462
polymer and the cationic sites on the methylene blue and rhodamine B. As Gg is an anionic
463
polysaccharide with abundant reactive hydroxyl and carboxylate ions (Aspinall, Hirst, &
Adsorption of cationic dyes from the aqueous solution
Page 19 of 37
464
Wickstrom, 1955; Aspinall, 1980), dye molecules can be easily attached to the binding sites
465
of the Gg-cl-P(AAm-AN) hydrogel polymer.
466
4.
468
A hydrogel-forming polymer of Gg with the co-polymer mixture of P(AAm-co-AN) was
469
successfully synthesized and was found to be more thermally stable than Gg. The synthesized
470
hydrogel polymer exhibited a maximum swelling capacity of 921% in neutral medium at 60
471
°C after 16 h. The Gg-cl-P(AAm-co-AN) hydrogel polymer also absorbed saline water from
472
different saline water-petroleum fraction emulsions, and the maximum absorption of saline
473
water was observed in the petroleum ether emulsion. Furthermore, the Gg-cl-P(AAm-co-AN)
474
hydrogel polymer was found to exhibit very good flocculation characteristics, with the
475
maximum flocculation efficiency observed with a 15 mg·l-1 polymer dose in acidic medium
476
at 40 °C. In soil composite, the Gg-cl-P(AAm-co-AN) hydrogel polymer was found to
477
degrade 89.47% within 60 days, and changes in surface morphology and chemical
478
composition were observed by SEM and FTIR. Moreover, The Gg-cl-P(AAm-AN) hydrogel
479
polymer successfully removed more than 98% of cationic dyes from aqueous solution. This
480
feasibility study confirmed that the hydrogel polymer synthesized here could be a good
481
adsorbent for the removal of cationic dyes from industrial waste water.
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Conclusions
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483
Acknowledgments
484
The authors are grateful to the National Research Foundation, South Africa for awarding a
485
postdoctoral research fellowship to Dr. Hemant Mittal. The authors (HM, AM, SSR) would
486
also like to thank the Department of Science and Technology and the Council for Scientific
487
and Industrial Research, South Africa for their financial support.
488
Page 20 of 37
Supplementary information
490
The supplementary document reports the TGA/DTA/DTG of the Gg-cl-P(AAM-co-AN)
491
hydrogel polymer and presents a comparison of the percentage thermal decomposition of the
492
Gg and Gg-cl-P(AAm-co-AN) hydrogel polymers.
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Synthetic Metals, 187, 9-16.
633
Zhang, X.Z., Yang, Y.Y., Chung, T.S., & Ma, K.X., (2001). Preparation and characterization
634
of fast response macroporous poly(N-isopropylacrylamide) hydrogels. Langmuir, 17, 6094-
635
6099.
and
flocculation
characteristics
of
ip t
Synthesis
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(2008).
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&Xie,
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636
D.,
637
638
639
640
Page 27 of 37
641
Captions to the Figures and Tables
642
Table 1:
emulsions. Table 2:
to the soil composting method. Fig. 1(a-c):
Effects of (a) time, (b) temperature and (c) pH on the Ps of the Gg-cl-P(AAm-
an
Fig. 2(a-c):
us
an image of a flower pot containing soil compost for biodegradation studies.
647
648
(a-b) Images of the sludge treatment plant at the NIT, Jalandhar, India and (c)
cr
645
646
Percentage weight loss of the Gg-cl-P(AAm-co-AN) hydrogel polymer subjected
ip t
643
644
Percentage removal of saline from different petroleum fraction-saline
co-AN) hydrogel polymer.
649
Effects of (a) time, (b) temperature, (c) pH on turbidity.
Fig. 3(a-c):
651
Figs. 4(a-d): FTIR spectra of (a) the Gg-cl-P(AAm-co-AN) hydrogel polymer, (b) the Gg-cl-
d
P(AAm-co-AN) hydrogel polymer in stage I of biodegradation (c) the Gg-cl-
Ac ce pt e
652
P(AAm-co-AN) hydrogel polymer in stage II of biodegradation, (d) the Gg-cl-
653
P(AAm-co-AN) hydrogel polymer in stage III of biodegradation.
654
655
Figs. 5(a-d): SEM images of (a) the Gg-cl-P(AAm-co-AN) hydrogel polymer, (b) the Gg-clP(AAm-co-AN) hydrogel polymer in stage I of biodegradation (c) the Gg-cl-
656
P(AAm-co-AN) hydrogel polymer in stage II of biodegradation, (d) the Gg-cl-
657
P(AAm-co-AN) hydrogel polymer in stage III of biodegradation.
658
659 660
M
650
Fig. 6:
Adsorption of methylene blue and rhodamine B using the Gg-cl-P(AAm-co-AN) hydrogel polymer.
661
Page 28 of 37
672
Highlights •
Hydrogel polymer of Gum ghatti with acrylamide-co-acrylonitrile was synthesized
674
•
Hydrogel polymer degraded 89.47% after 60 days using soil composting method
675
•
Hydrogel polymer showed pH and temperature responsive swelling studies
676
•
Hydrogel polymer showed maximum flocculation efficiency in acidic medium
677
•
Hydrogel polymer removed 98% of MB and RhB from aqueous solution
cr
678
ip t
673
Ac ce pt e
d
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an
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679
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Ac
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Figure(s)
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Figure(s)
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Figure(s)
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Percentage removal of saline from different petroleum fraction-saline emulsions
us
Table 1:
cr
ip t
Table(s)
Sample
Percentage saline removal Petrol-saline
Pet. ether-saline
% removal
±SD
±SE
% removal
±SD
±SE %removal ±SD
±SE
% removal
±SD
±SE
21.0
1.27
0.73
18.7
0.51
0.30
0.30
25.5
0.87
0.50
M
Gg-cl-P(AAm-co-AN)
Diesel-saline
an
Kerosene-saline
23.5
0.52
Ac c
ep te
d
whereas, no. of replications = 03; amount of each emulsion taken= 100ml (1:1 v/v); weight of superabsorbent taken for each emulsion= 1.0g
Page 36 of 37
cr
ip t
Table(s)
Table 2: Percentage weight loss of Gg-cl-P(AAm-co-AN) hydrogel polymer using soil composting method Percentage weight loss at different time intervals (Days) 10
15
20
11.53
16.25
21.95
33.96
25
30
35
40
45
50
55
60
37.50
41.86
48.93
57.62
65.37
73.05
77.14
89.47
Ac c
ep te
d
M
an
Gg-cl-P(AAm-co-AN)
5
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Sample Code
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S0144-8617(14)00798-X http://dx.doi.org/doi:10.1016/j.carbpol.2014.08.029 CARP 9166
To appear in: Received date: Revised date: Accepted date:
13-6-2014 30-7-2014 7-8-2014
Please cite this article as: Mittal, H., Jindal, R., Kaith, B. S., Maity, A., and Ray, S. S.,Synthesis and flocculation properties of gum ghatti and poly(acrylamideco-acrylonitrile) based biodegradable hydrogels, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.08.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Submitted to Carbohydrate Polymers
2
Revised version
3
Synthesis and flocculation properties of gum ghatti and poly(acrylamide-
5
co-acrylonitrile) based biodegradable hydrogels
7
Hemant Mittala,b *, Rajeev Jindalc, Balbir Singh Kaithc, Arjun Maityb and Suprakas Sinha
8
Raya, b *
Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, South Africa b
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d
c
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Department of Chemistry, National Institute of Technology Jalandhar 14401, Punjab, India
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M
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DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa
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*Corresponding authors: Fax: +27128412229; E-mails: [email protected] (HM)
17
and [email protected] (SSR)
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Page 1 of 37
Abstract
23
This article reports the development of biodegradable flocculants based on graft co-polymers
24
of gum ghatti (Gg) and a mixture of acrylamide and acrylonitrile co-monomers (AAm-co-
25
AN). The hydrogel polymer exhibited an excellent swelling capacity of 921% in neutral
26
medium at 60°C. The polymer was used to remove saline water from various petroleum
27
fraction-saline water emulsions. The flocculation characteristics of the hydrogel polymer
28
were studied in turbid kaolin solution as a function of the amount of polymer and the solution
29
temperature and pH. Biodegradation studies of hydrogel polymer were conducted using the
30
soil composting method, and the degradation process was constantly monitored using
31
scanning electron microscopy and Fourier transform infrared spectroscopy techniques. The
32
results demonstrated an 89.47% degradation of the polymer after 60 days. Finally, the
33
hydrogel polymer adsorbed 98% of cationic dyes from the aqueous solutions.
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Keywords: gum ghatti; hydrogel; swelling; flocculation; biodegradation; adsorption
36
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1.
Introduction
38
In recent decades, synthetic polymers have been extensively used by many industries because
39
of their excellent mechanical and physical properties. However, the main disadvantage of
40
synthetic polymers is their non-degradable nature, which makes their disposal difficult.
41
Recently, researchers have made a variety of attempts to replace non-degradable polymers
42
with biodegradable polymers (Averous, & Pollet, 2013; Hermanova, et al., 2013; Okada et
43
al., 2002; Liang, et al., 2013; Zare, Lakouraj, & Mohseni, 2014). Biodegradation is the
44
process of the fragmentation of long polymer chains into low molecular weight compounds in
45
the presence of microorganisms. Biodegradation of polymers depends upon several factors
46
including the chemical and physical properties of polymers, the composition of the final
Page 2 of 37
product and the length and crystallinity of the polymer chains (Abrusci, Marquina, &
48
Catalina, 2007; Nageotte, Pester, Pradet, & Bayard, 2006). The experimental conditions
49
including temperature and humidity also influence the biodegradability of polymers.
50
While biopolymers are highly biodegradable, they tend to have poor properties compared to
51
synthetic polymers. Biopolymers also have some potential advantages over synthetic
52
polymers, including their excellent biocompatibility, their eco-friendly nature, low cost,
53
abundant availability, and suitability for use in many biomedical applications (Jenkins,
54
Samuel, & Hudson, 2001; Park, Park, & Kim, 2006). Moreover, the inherent properties of
55
biopolymers can be improved through a variety of techniques including graft co-
56
polymerization. Graft co-polymerization is the most widely studied technique to improve the
57
inherent properties of biopolymers, and it has mainly been used for polysaccharide-based
58
biopolymers. Recently, many studies have been carried out on the graft co-polymerization of
59
gum polysaccharides such as gum ghatti, gum xanthan, gum arabic and guar gum (Kaith,
60
Ranjta, & Kumar, 2008; Mittal, Fosso-Kankeu, Mishra, & Mishra, 2013; Mittal, Ballav, &
61
Mishra, 2014; Sand, Yadav, Mishra, Tripathi, & Behari, 2011; Sanghi, Bhattacharya, &
62
Singh, 2006), and the resulting graft co-polymers have potential applications in the food
63
industry, as drug delivery devices, in soft tissue engineering and waste water treatment
64
(Mariano, El Kissi, & Dufresne, 2014; Mittal, & Mishra, 2014; Sand, Yadav, & Behari, 2010;
65
Sarkar, Mandre, Panda, & Pal, 2013; Thakur, Thakur, Raghavan, & Kessler, 2014).
66
The anionic polysaccharide gum ghatti (Gg) is an exudate of the Anogeissus latifolia tree,
67
which belongs to the Combretaceae family. The structure of Gg is composed of sugars such
68
as L-arabinose, D- galactose, D-mannose, D-xylose and D-glucoronic acid in a 48:29:10:5:10
69
molar ratio (Aspinall, Hirst, & Wickstrom, 1955). The molecular structure of Gg contains
70
alternating 4-O-substituted and 2-O-substituted α-D-mannopyranose units and chains of 1Æ6
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Page 3 of 37
linked β-D-galactopyranose units with side chains that are most frequently single L-
72
arabinofuranose residues (Aspinall, 1980; Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia,
73
2103) (Fig. S1, supplementary data).
74
The process of removing different organic and inorganic impurities from waste water by
75
converting suspended particles into heavy flocs is called flocculation. Flocculation is a highly
76
efficient technique for the purification of industrial and domestic wastewater and the
77
thickening of sludge. Flocculants can be either inorganic, usually called coagulant agents
78
(such as aluminum sulfate and ferric chloride), or polymeric materials (Ghimici, Constantin,
79
& Fundueanu, 2010). The properties of polymer-based flocculants can be easily tailored to
80
specific applications. Moreover, the flocculation capacity of polymer-based flocculants is
81
much higher than that of inorganic flocculants (Singh et al., 2000). Polymer-based flocculants
82
can be synthetic (Ghimici, Dranca, Dragan, Lupascu, & Maftuleac, 2001; Nasser, & James,
83
2006) or natural (Ghimici, & Constantin, 2011; Pal, Sen, Karmakar, Mal, & Singh, 2005;
84
Simon, Picton, Le Cerf, & Muller, 2005). Synthetic polymer-based flocculants exhibit better
85
performance than natural polymer-based flocculants. However, natural polymer-based
86
flocculants have their own advantages including low cost, biodegradability and non-toxicity
87
(Ghimici, Constantin, & Fundueanu, 2010). Therefore, it is critical to develop flocculant
88
materials with the advantages of both synthetic and natural polymers. Recently, a great deal
89
of work has been reported aiming to develop flocculants through the graft co-polymerization
90
of polysaccharides with synthetic vinyl monomers, such as poly(acrylic acid) and
91
poly(acrylamide) (Chang, Hao, & Duan, 2008; Mittal, Mishra, Mishra, Kaith, Jindal, &
92
Kalia, 2103; Singh et al., 2000; Tian, & Xie, 2008; Wan, Li, Wang, Chen, & Gu, 2007).
93
The purification of industrial effluent water is one of the greatest challenges faced by
94
developing countries. Industries producing batteries, textiles, paper, cosmetics and ink
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generate huge amounts of wastewater contaminated with various types of dyes. It is very
96
difficult to degrade dyes in sludge treatment plants because most dyes are non-degradable in
97
nature (Kumari, & Abraham, 2007). Adsorption is one of the most efficient techniques used
98
for the treatment of dye-contaminated water. In this direction, gum polysaccharide-based
99
graft co-polymers have demonstrated their potential to efficiently adsorb dyes from
100
wastewater (Ghorai, Sarkar, Raoufi, Panda, & Schonherr, 2014; Mittal, & Mishra, 2014;
101
Mittal, Ballav, & Mishra, 2014).
102
While researchers have tested these polymers for their flocculation characteristics, swelling
103
properties and ability to adsorb pollutants from contaminated water, these polymers do not
104
exhibit these key properties, resulting in poor performance. Furthermore, synthesized graft
105
co-polymers exhibited considerable resistance to biodegradation (Mishra, Sand, Mishra,
106
Yadav, & Behari, 2010; Sand, Yadav, & Behari, 2010; Sand, Yadav, Mishra, Tripathy, &
107
Behari, 2011). In previous studies, we also reported the swelling properties, flocculation
108
characteristics and biodegradation of cross-linked graft co-polymers of Gg with AAm (Mittal,
109
Mishra, Mishra, Kaith, & Jindal, 2103) and with the co-polymer mixture of AAm with the
110
hydrophilic vinyl monomer acrylic acid (AA) (Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia,
111
2103). The current study reports the effect of a co-polymer mixture of AAm with a
112
hydrophobic vinyl monomer, i.e., acrylonitrile (AN) on the flocculation characteristics,
113
swelling properties, thermal stability, adsorption efficiency and biodegradation behavior of a
114
cross-linked graft co-polymer of Gg.
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2.
Experimental
117
2.1.
Materials
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AAm, AN, Gg, N, N’-methylene-bis-acrylamide (MBA), potassium persulfate (KPS),
119
ascorbic acid (ABC), petroleum ether, methylene blue, rhodamine B and kaolin were
120
purchased from Merck Chemicals (India) and used as received. Petrol and diesel were
121
purchased from Indian Oil Corporation Limited, India. All chemicals were of analytical grade
122
and were used as received without further purification. All solutions were prepared in triple-
123
distilled water. Flocculation studies were conducted on a Micro 1000 IR Turbidimeter (HF
124
Scientific Incorporation, USA). Stock solutions of dyes (1000 mg.l-1) were prepared by
125
dissolving the appropriate amount of dye in sterile distilled water; adequate volumes of the
126
stock solutions were further diluted in distilled water to prepare experimental solutions.
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an
127 128
2.2.
129
The cross-linked hydrogel polymer of Gg with the co-polymer mixture of P(AAm-co-AN)
130
was prepared using graft co-polymerization (Jindal, Kaith, & Mittal, 2010). At first, 1 g of Gg
131
was added to 30 ml distilled water and left undisturbed for 24 h to obtain a homogeneous
132
mixture of gum polysaccharide in water. After 24 h, a mixture of KPS (30 mg) and ABC (20
133
mg) was added to the reaction mixture with constant stirring, followed by the addition of
134
MBA (50 mg). The reaction temperature was maintained at 60 °C. The monomer mixture of
135
AAm (1.0 g) and AN (2.0 ml) was added to the reaction vessel. The reaction was allowed to
136
continue for the next 6 h. After that, the reaction vessel was removed and cooled to room
137
temperature. The homopolymers polyacrylamide and polyacrylonitrile and the unreacted co-
138
polymer P(AAm-co-AN) were removed by successive extractions with acetone and N,N’-
139
dimethylformamide, respectively. Finally, the synthesized hydrogel was dried in a hot air
140
oven at 60 °C until a constant weight was achieved.
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Synthesis of the Gg-cl-P(AAm-co-AN) hydrogel polymer
141 142
2.2.1. Characterization
Page 6 of 37
Changes in the thermal behavior of Gg after graft co-polymerization and cross-linking with
144
P(AAm-co-AN) and MBA were studied using TGA/DTA/DTG (TG/DTA 6300, SII
145
EXSTAR 6000, SII Nanotechnology Incorporation, Japan) in air at a heating rate of 10 °C
146
per minute. The FTIR spectra of the Gg and the Gg-cl-P(AAm-co-AN) hydrogel polymer
147
before degradation and at various stages of degradation were recorded on a PERKIN ELMER
148
RXI spectrophotometer using KBr pellets (Sigma Aldrich) in the spectral range of 4000-400
149
cm-1 with a resolution of 4 cm-1. The morphological change of the hydrogel polymers at
150
various stages of biodegradation were examined using SEM (LEO, 435 VF, LEO Electron
151
Microscopy Ltd., Japan). As all samples were non-conductive, they were coated with gold to
152
introduce conductivity.
153
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143
154
2.3.
155
As the ability to swell in water is one of the most important features of hydrogels, it is very
156
important to determine the swelling capacity of the synthesized hydrogel polymer. The
157
swelling efficiency of the Gg-cl-P(AAm-AN) hydrogel polymer was investigated at various
158
time intervals (4, 8, 12, 16, 20 and 24 h), temperatures (30, 40, 50, 60 and 70 °C) and pH
159
values (3.0 to 13.0) in deionized water. For each experiment, several 100 mg of samples of
160
hydrogel polymer were each immersed in 100 ml of deionized water. After a particular time
161
interval, the samples were removed, gently wiped and weighed. The percentage swelling (Ps)
162
was then calculated using Eq. (1) (Kaith, Jindal, Mittal, & Kumar, 2012):
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Swelling studies in deionized water
Ps = 166
W s − Wd × 100 Wd
(1)
167
where Ws and Wd are the weights of the swelled and dry hydrogel polymer samples,
168
respectively.
Page 7 of 37
169
After the swelling time was optimized, the effect of temperature on Ps was studied at a pre-
170
optimized time, and then the pH of the swelling medium was optimized to obtain the
171
maximum Ps at the pre-optimized time and temperature.
172
2.3.1.
174
Selective absorption of saline from various petroleum fraction-saline
ip t
173
emulsions
The separation of saline water from crude petroleum during their extraction from the sea is
176
challenging because current processes are highly tedious and expensive. The hydrogel
177
polymers synthesized here were tested for the selective absorption of saline water from
178
various petroleum fraction-saline emulsions. A 1% NaCl solution was used as saline water
179
with various petroleum fractions such as kerosene, diesel, petrol and petroleum ether. From
180
each emulsion, 50 ml (i.e., 25 ml of saline water and 25 ml of the petroleum fraction) was
181
collected in a 250 ml conical flask. A stable emulsion was prepared by adding 10 ml tween-
182
20, and the flasks were shaken using an automatic shaker machine (LABCO, India). Once a
183
stable emulsion was obtained, 100 mg of hydrogel polymer was added. After 16 h, the flasks
184
were removed from the shaker and allowed to stand undisturbed for 48 h to break up the
185
emulsions. The petroleum fractions were then separated using a separating funnel, and the
186
remaining unabsorbed volume of saline water was measured.
187
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188
2.4.
189
Flocculation characteristics of the Gg-cl-P(AAm-AN) hydrogel polymer were investigated
190
using the standard jar test (Jha, Agarwal, Mishra, & Rai, 2001; Mittal, Mishra, Mishra, Kaith,
191
& Jindal, 2013). For each experiment, a volume of 500 ml of the 50 ppm kaolin solution was
192
placed in 1000 ml and agitated at 180 rpm for 10 min. Initially, a blank reading was taken
193
followed by the addition of hydrogel polymer to the kaolin suspension. Readings were taken
Flocculation studies
Page 8 of 37
194
every 10 min until almost constant turbidity was obtained. Different doses of the hydrogel
195
polymer were used to determine the optimal dose to maximize flocculation capacity. The
196
flocculation temperature was then optimized, followed by the solution pH.
197 198
2.5.
199
The biodegradation of the Gg-cl-P(AAm-AN) hydrogel polymer was studied using the soil
200
composting method (Kaith, Jindal, Maiti, & Jana, 2010). Biodegradation studies were
201
performed in flower pots, and the compost was collected from the waste water effluent
202
discharge plant of the National Institute of Technology, Jalandhar, India (Fig. 1). For each
203
experiment, rectangular samples (2 x 2 x 1.5 cm) were prepared and buried in soil compost in
204
a flower pot at a depth of 2 cm, 3 cm apart from each other. Microbe rich water from the
205
plant was added to the compost on alternating days to compensate for water loss caused by
206
evaporation. The progress of the degradation process was examined every 5 days. Every 5
207
days, the samples were extracted from the compost, carefully cleaned, dried at 50 °C in a
208
vacuum oven and weighed.
209
The percentage weight loss at each stage of degradation was calculated using Eq. (2) (Kaur,
210
Bhalla, Deepika, & Gautam, 2009):
ip t
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d
Ac ce pt e
211
Biodegradation studies
Weight loss (%)
=
212
where Wi is the initial weight of the hydrogel polymer and Wt is the weight after a particular
213
time interval.
214
The progress of degradation was also evaluated by studying the changes in physical
215
appearance and chemical composition using SEM and FTIR, respectively.
216 217
Page 9 of 37
218
2.6.
219
Adsorption studies of the cationic dyes methylene blue and rhodamine B were carried out in
220
batch mode. A 50 mg·l-1 solution of each dye (50 ml) was first placed in 100 ml glass bottles.
221
Various amounts of the Gg-cl-P(AAm-co-AN) hydrogel polymer were added to each dye
222
solution. The bottles were placed in a thermostatic water bath and shaken for 24 h at 200 rpm.
223
After 24 h, the bottles were removed from the water bath and the dye solutions were filtered
224
using a 0.45 µm cellulose acetate syringe filter. The concentration of unadsorbed dye
225
remaining in the solution was analyzed using a UV/vis spectrophotometer (Shimadzu, Japan
226
UV-2450, UV-vis spectrophotometer) at 554 nm as the λmax of rhodamine B and 664 nm as
227
the λmax of methylene blue.
228
calculated using Eq. (3) (Sadeghi, Azhdari, Arabi, & Moghaddam, 2012):
M
an
The adsorption efficiency or the percentage removal was
d
229
231
3.
232
3.1.
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233
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Adsorption of methylene blue and rhodamine B from the aqueous solution
Results and Discussion
Thermogravimetric analysis, differential thermal analysis and differential thermo-gravimetric analysis (TGA/DTA/DTG)
234
The initial decomposition temperature (IDT) of the Gg-cl-P(AAm-co-AN) hydrogel polymer
235
was found to be 133.6 °C (Fig. S2, Supplementary Data), which was much lower than the
236
IDT of the Gg, i.e., 206.9 °C (Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). In contrast, the
237
final decomposition temperature (FDT) of the Gg-cl-P(AAm-co-AN) hydrogel polymer
238
(681.8 °C) was found to be much higher than the FDT of the Gg (521.6 °C) (Mittal, Mishra,
239
Mishra, Kaith, & Jindal, 2013). The graft co-polymerization of the Gg with the binary
240
monomer mixture of P(AAm-co-AN) decreases the IDT. This may be associated with the
241
degradation of P(AAm-co-AN) chains as a result of the formation of the imide group with the
Page 10 of 37
evolution of ammonia. Furthermore, as the formation of amide groups increased the FDT of
243
the graft co-polymer, a higher FDT was observed in the case of the Gg-cl-P(AAm-co-AN)
244
hydrogel polymer compared to Gg (Behari, Pandey, Kumar, & Taunk, 2001). The Gg-cl-
245
P(AAm-co-AN) hydrogel polymer degradation curve also exhibited three stages of
246
decomposition ranging from 133.6-276.8 °C (weight loss, 17.3%), 276.8-436.2 °C (weight
247
loss, 18.3%) and 436.2-681.1 °C (weight loss, 61.3%). In contrast, the TGA of the Gg
248
exhibited two stage decomposition over the temperature ranges from 206.9-331.4 °C (weight
249
loss, 46.6%) and 331.4-521.6 °C (weight loss, 31.2%). Moreover, a detailed study of the
250
percentage weight loss at different temperatures indicated that in Gg, a certain percentage
251
weight loss occurred at lower temperatures compared to the Gg-cl-P(AAm-co-AN) hydrogel
252
polymer (Table S1, Supplementary Data).
cr
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253
ip t
242
The DTA of the Gg exhibited two exothermic peaks at 483.2 °C (203 µV) and 492.7 °C (154
255
µV), whereas the DTA of the Gg-cl-P(AAm-co-AN) hydrogel polymer exhibited exothermic
256
peaks at 280.8 °C (102.2 μV) and 669.9 °C (136.2 μV). DTA peaks were observed at much
257
higher temperatures with less heat evolved in the DTA of the Gg-cl-P(AAm-co-AN) (Fig. S2,
258
Supplementary Data) compared to the Gg (Mittal, Mishra, Mishra, Kaith, & Jindal, 2013).
259
The Gg-cl-P(AAm-co-AN) hydrogel polymer exhibited two DTG peaks at 280.8 °C with a
260
mass loss rate of 2.223 mg/min and at 669.4 °C with a mass loss rate of 1.01 mg/min (Fig.
261
S2, Supplementary Data). In contrast, the DTG of the Gg exhibited decompositions at 299.1
262
°C and 479.0 °C with mass loss rates of 1.463 and 2.23 mg/min, respectively (Mittal, Mishra,
263
Mishra, Kaith, & Jindal, 2013). DTG studies showed that the rate of thermal decomposition
264
was higher in Gg than in the Gg-cl-P(AAm-co-AN) hydrogel polymer. Thus,
265
TGA/DTA/DTG studies suggest that the thermal stability of Gg improved after including
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266
P(AAm-co-AN) chains in the backbone polymer through graft co-polymerization and cross-
267
linking.
268
3.2.
270
Swelling studies of the Gg-cl-P(AAm-co-AN) hydrogel polymer in deionized water
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271
3.2.1.
272
The effect of swelling time on Ps was studied at various time intervals (4, 8, 12, 16, 20 and 24
273
h) (Fig. 2(a)). Initially, the Ps was found to increase with increasing swelling time, reaching a
274
maximum Ps (669%) after 16 h. Further increases in swelling time did not lead to any further
275
increases in swelling capacity, and almost a constant value of Ps was observed. This may be
276
because after reaching maximum swelling, the three dimensional porous network of the Gg-
277
cl-P(AAm-co-AN) hydrogel polymer becomes fully saturated, with no more active sites
278
available to accommodate water molecules. Therefore, no increase in swelling capacity was
279
observed after 16 h swelling time.
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Effect of time on Ps
281
3.2.2.
282
The effect of temperature on Ps was studied at various temperatures (30, 40, 50, 60 and 70
283
°C) at the pre-optimized swelling time (16 h) (Fig. 2(b)). Initially, swelling capacity was
284
found to increase with increasing temperature up to 60 °C, reaching a maximum value of Ps
285
(921%). However, with further increases in temperature, the Ps of the Gg-cl-P(AAm-co-AN)
286
hydrogel polymer decreased remarkably, reaching 863% at 70 °C. This suggests that
287
increases in temperature initially increased the elasticity of the cross-linked hydrogel polymer
288
network, resulting in a larger Ps, but above the optimal temperature the polymer network
289
collapsed, leading to desorption with further increases in temperature (Zhang, Yang, Chung,
290
& Ma, 2001). Moreover, the hydrophilic groups of the hydrogels formed hydrogen bonds
Effect of temperature on Ps
Page 12 of 37
with water molecules. These bonds cooperatively formed a stable shell of hydration around
292
the hydrophilic groups, increasing the uptake of water and thus increasing Ps. However, the
293
associated interactions among the hydrophilic groups released the entrapped water molecules
294
from the hydrogel network at higher temperatures. Therefore, a lower Ps value was observed
295
at higher temperatures (Feil, Bae, Feijen, & Kim, 1992; Inomato, Goto, & Saito, 1990).
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296 297
3.2.3.
298
Solution pH was found to have a profound effect on the swelling of hydrogel polymers, as
299
reported by a number of researchers (Mahdavinia, Pourjavadi, Hosseizadeh & Zohurian,
300
2004; Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). The effect of pH on the Ps of the Gg-cl-
301
P(AAm-co-AN) hydrogel polymer was studied in solutions with pH values ranging from 3.0
302
to 13.0 at the pre-optimized swelling time and temperature (Fig. 2(c)). A higher value of Ps
303
was observed in the solution at neutral pH (921%) compared to acidic and alkaline pH
304
solutions. The swelling capacity of the hydrogel polymer increased with solution pH. This
305
type of swelling behavior can be explained based on osmotic swelling pressure theory. In the
306
case of dilute electrolyte solutions, the swelling capacity of the hydrogel polymers depends
307
on osmotic swelling pressure (πion), given as (Bajpai, 1999):
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Effect of pH on Ps
πion = RT∑ (Cig - Cis)
(4)
309
where Cig and Cis are the molar concentrations of mobile ions in the swollen gel and external
310
solution, respectively. R and T are the gas constant and absolute temperature, respectively.
311
In the solutions of neutral medium, the concentration of mobile ions (Cis) in swelling medium
312
became very small, increasing the value of πions. In contrast, in acidic solutions, most of the
313
carboxylate ions were protonated (Mahdavinia, Pourjavadi, Hosseizadeh & Zohurian, 2004),
314
decreasing the concentration of mobile ions within the swollen gel (Cig), leading to a reduced
Page 13 of 37
315
value of πions. Furthermore, in alkaline solutions, –COOH groups completely dissociated and
316
Na+ and –OH ions were present at very high concentration, reducing the values of πions and Ps
317
(Mahdavinia, Pourjavadi, Hosseizadeh & Zohurian, 2004; Mittal, Mishra, Mishra, Kaith, &
318
Jindal, 2013).
3.2.4.
321
Selective absorption of saline from different petroleum fraction-saline emulsions
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Initially, the swelling behavior of the Gg-cl-P(AAm-co-AN) hydrogel polymer was tested in
323
different petroleum fractions, i.e., petrol, kerosene, diesel and petroleum ether, and the
324
hydrogel polymer did not absorb any of these petroleum fractions. The maximum saline
325
adsorption was observed in the petroleum ether-saline emulsion (25.5%) followed by the
326
petrol-saline emulsion (23.5%), the kerosene-saline emulsion (21.0%) and the diesel-saline
327
emulsion (18.7%) (Table 1). This adsorption trend can be attributed to the length of
328
hydrocarbon chains present in the various petroleum fractions. Petroleum fractions with
329
larger hydrocarbon chains have large hydration shells, impeding the absorption of water
330
molecules by the hydrogel polymer. As petroleum ether has the fewest carbon atoms (C15), the maximum adsorption
332
of saline was observed in the petroleum ether-saline emulsion, followed by the petrol-saline
333
emulsion, the kerosene-saline emulsion and the diesel-saline emulsion (Mittal, Mishra,
334
Mishra, Kaith, Jindal, & Kalia, 2013).
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335 336 337 338
3.3.
Characterization of the flocculation of the Gg-cl-P(AAm-co-AN) hydrogel polymer
3.3.1. Effect of polymer dose on flocculation efficiency
Page 14 of 37
The effect of the amount of the Gg-cl-P(AAm-co-AN) hydrogel polymer on the turbidity of
340
the kaolin suspension was studied in the presence of various masses of hydrogel polymer (10,
341
15, 20 and 25 mg.l-1) in a neutral medium. Initially, the turbidity of the kaolin suspension
342
decreased with increasing amounts of hydrogel polymer, reaching a minimum value of 2144
343
NTU with 15 mg·l-1 polymer (Fig. 3(a)). However, further increases in the amount of
344
hydrogel polymer resulted in increases in turbidity. The gradual initial decrease in the
345
turbidity of the kaolin suspension was due to the formation of particle-polymer-particle
346
complexes between the kaolin particles and the Gg-cl-P(AAm-co-AN) hydrogel polymer,
347
where the hydrogel polymer acted as a bridge between different suspended kaolin particles
348
(Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia, 2013; Tian, & Xie, 2008). Kaolin particles
349
attached to different functional groups present on the polymer chain as soon as they came
350
into contact with the hydrogel polymer, leading to the formation of extended polymer chains.
351
In the presence of lower amounts of polymer, insufficient functional groups were available to
352
act as bridges between different clay particles. However, as the polymer dose was increased
353
the number of functionalities increased, increasing the number of bridges accessible for the
354
formation of particle-polymer-particle complexes, decreasing the turbidity value. Therefore,
355
increasing the amount of hydrogel polymer initially resulted in a gradual decrease in
356
turbidity. Further increases in the amount of hydrogel polymer beyond the optimum increased
357
the turbidity of the kaolin suspension because beyond the optimum dose, kaolin particles will
358
envelope between different polymer chains, thus failing to come into contact with the
359
functional groups of the hydrogel polymer (Xie, Feng, & Cao, Xia, & Lu, 2007). Kaolin
360
particles in suspension will be re-stabilized, resulting in a higher turbidity value. Therefore,
361
particle-polymer-particle complexes will not be formed in the presence of excessive amounts
362
of hydrogel polymer, resulting in a higher turbidity value (Ghimici, & Nichifor, 2010; 2012;
363
Ghimici, Constantin, & Fundueanu, 2012).
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Page 15 of 37
3.3.2. Effect of temperature on flocculation efficiency
365
The effect of temperature on the flocculation efficiency of the Gg-cl-P(AAm-co-AN)
366
hydrogel polymer is shown in Fig. 3(b). Initially, turbidity was found to decrease with
367
increasing temperature, reaching a minimum value of 1915 NTU at 40 °C. Further increases
368
in temperature beyond the optimum value increased the turbidity of the kaolin solution. The
369
initial decrease in the turbidity of the kaolin solution was associated with the formation of
370
particle-polymer-particle complexes and was supported by the initial increase in temperature.
371
However, at higher temperatures, the kinetic energy of hydrogel polymer molecules was
372
increased, resulting in the re-dispersion of kaolin particles and a higher turbidity value
373
(Mittal, Mishra, Mishra, Kaith, Jindal, & Kalia, 2013).
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M
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3.3.3. Effect of pH on flocculation efficiency
376
The effect of the solution pH on the flocculation efficiency of the Gg-cl-P(AAm-co-AN)
377
hydrogel polymer was examined in kaolin solutions of various pH values at the pre-optimized
378
polymer dose and temperature (Fig. 3(c)). The minimum turbidity of the kaolin solution was
379
observed at acidic pH, and the turbidity of the kaolin solution increased with increasing
380
solution pH, with the maximum turbidity observed in the kaolin solution at alkaline pH. The
381
turbidity was lower in acidic solutions because under acidic conditions, the hydroxyl and
382
carboxylate groups present in the Gg-cl-P(AAm-co-AN) hydrogel polymer were protonated,
383
imparting a cationic charge to the overall hydrogel polymer network (Mittal, Mishra, Mishra,
384
Kaith, Jindal, & Kalia, 2013). Negatively charged kaolin particles were easily attached to
385
those cationic sites, forming larger flocks that easily settled, resulting in a reduction in the
386
turbidity of the kaolin solution. In alkaline solutions, amides are weakly acidic and resonance
387
stabilized (Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). Therefore, due to the anionic-
388
anionic repulsion between the functional groups of the Gg-cl-P(AAm-co-AN) hydrogel
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Page 16 of 37
389
polymer and kaolin particles, particle-polymer-particle complexes were not formed and little
390
decrease in the turbidity of the kaolin solution was observed.
391 392
3.4.
393
While biopolymers are well-suited for many applications due their biodegradable nature, they
394
also have serious shortcomings for many applications. Therefore, many biopolymers need to
395
be chemically modified without affecting the biodegradable nature of the biopolymer. Here,
396
we studied the effect of graft co-polymerization and crosslinking of Gg with the binary
397
monomer mixture of P(AAm-co-AN) on its biodegradability. In the Gg-cl-P(AAm-co-AN)
398
hydrogel polymer, the process of biodegradation starts with the backbone polymer, i.e., Gg.
399
The weight of the hydrogel polymer was measured every 5 days (Table 2). The
400
biodegradation process was found to be continuous in terms of the percentage weight loss of
401
the Gg-cl-P(AAm-co-AN) hydrogel polymer versus time. The Gg-cl-poly(AAm-co-AA)
402
hydrogel polymer degraded by up to 89.47% over 60 days. The polymer samples were both
403
enzymatically and chemically degraded. Furthermore, the secretion products of the microbes
404
present in the composting media were also involved in the degradation of the hydrogel
405
polymer, leading to the cleavage of chemical bonds and bacterial degradation (Kaith, Jindal,
406
Maiti, & Jana, 2010).
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407
Biodegradation studies of the Gg-cl-P(AAm-co-AN) hydrogel polymer
408
3.4.1. Evidence of bio-degradation
409
3.4.1.1.
410
The FTIR spectrum of the Gg-cl-P(AAm-co-AN) hydrogel polymer was recorded at different
411
stages of biodegradation (Figs. 4(a-d)). The FTIR of the Gg-cl-P(AAm-co-AN) hydrogel
412
polymer before biodegradation exhibited peaks at 1673.25 cm-1 (C=O stretching of amide I
413
band), 1453.57 cm-1 (NH in-plane bending of the amide II band),
Spectroscopic studies
1253.44 cm-1 (CN
Page 17 of 37
stretching vibrations of the amide III band), 749 cm-1 (OCN deformations of the amide IV
415
band) and 2240.78 cm-1 (C≡N of acrylonitrile) (Fig. 4(a)) in addition to the characteristic
416
peaks of Gg (Kaith, Jindal, Mittal, & Kumar, 2012). However, the FTIR spectra of degraded
417
samples at different stages of biodegradation showed peaks with reduced intensity and
418
different peak positions compared to the FTIR spectrum of the Gg-cl-P(AAm-co-AN)
419
hydrogel polymer before degradation. Peaks initially appearing at 1673.25 cm-1, 2240.78 and
420
1453.57 cm-1 were shifted, with reduced intensity at biodegradation stage I (Fig. 4(b)). This
421
may be due to the initiation of the breakdown of the cross-linked network. However, the
422
degradation of individual compounds such as Gg and P(AAm-co-AN) began in the second
423
stage of biodegradation (Fig. 4(c)). The major process in this stage was the degradation of
424
Gg, together with the shifting or complete disappearance of the peaks related to the Gg. Most
425
of the peaks initially appearing in the FTIR spectrum of the Gg-cl-P(AAm-co-AN) hydrogel
426
polymer were completely shifted or absent in the third and final stage of biodegradation (Fig.
427
4(d)). Thus, FTIR studies at different stages of biodegradation confirmed that the grafted
428
chains of P(AAm-co-AN) and Gg were biodegraded in the soil compost.
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430
3.4.1.2.
431
The changes in the morphology of the Gg-cl-P(AAm-co-AN) hydrogel polymer at different
432
stages of biodegradation were monitored using SEM (Figs. 5(a-d)). Before the degradation
433
process started, the surface of the Gg-cl-P(AAm-co-AN) hydrogel polymer was found to be
434
smooth and homogeneous, with a continuous and regular morphology (Fig. 5(a)). However,
435
after the biodegradation process started, i.e., at the early stages of biodegradation
436
(biodegradation stage-I), the number of cracks on the surface of the hydrogel polymer
437
increased, and the homogeneous surface started becoming a fibrillar and heterogeneous
438
surface (Figs. 5(b-c)). Some fractures and fissures also started to appear on the surface of the
Morphology studies
Page 18 of 37
hydrogel polymer at the early stages of biodegradation, which confirmed the initiation of the
440
degradation process. These fractures and fissures on the surface of the hydrogel polymer may
441
be due to the initiation of the breakage of cross-links between different polymer chains,
442
beginning the process of degrading individual components of the hydrogel polymer (Mittal,
443
Mishra, Mishra, Kaith, Jindal, & Kalia, 2013). In the third and final stage of biodegradation,
444
the size and number of these fissures and fractures were drastically increased, and the
445
polymer surface became completely rough and heterogeneous due to the complete fractured
446
surface and the complete degradation of the individual components of the Gg-cl-P(AAm-co-
447
AN) hydrogel polymer (Fig. 5(d). This may be due to the complete breakage of covalent
448
bonds between different polymeric chains through chemical and enzymatic degradation by
449
the secreted products of different micro-organisms (Mittal, Mishra, Mishra, Kaith, Jindal, &
450
Kalia, 2013; Mittal, Mishra, Mishra, Kaith, & Jindal, 2013). Thus, SEM studies predicted that
451
enzymes secreted by micro-organisms were responsible for the degradation of the Gg-cl-
452
P(AAm-co-AN) hydrogel polymer.
Ac ce pt e
453
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454
3.5.
455
The Gg-cl-P(AAm-co-AN) hydrogel polymer was successfully used for the adsorption of
456
methylene blue and rhodamine B from aqueous solutions, and the results are shown in Fig. 6.
457
The percentage adsorption was found to increase as the amount of hydrogel polymer
458
increased. Almost 98% of methylene blue and rhodamine B were found to adsorb to 0.4 g·l-1
459
and 0.8 g·l-1 of the hydrogel polymer, respectively. We believe that the driving force for the
460
high adsorption of cationic dyes using the Gg-cl-P(AAm-co-AN) hydrogel polymer is the
461
electrostatic interaction between negatively charged functional groups of the hydrogel
462
polymer and the cationic sites on the methylene blue and rhodamine B. As Gg is an anionic
463
polysaccharide with abundant reactive hydroxyl and carboxylate ions (Aspinall, Hirst, &
Adsorption of cationic dyes from the aqueous solution
Page 19 of 37
464
Wickstrom, 1955; Aspinall, 1980), dye molecules can be easily attached to the binding sites
465
of the Gg-cl-P(AAm-AN) hydrogel polymer.
466
4.
468
A hydrogel-forming polymer of Gg with the co-polymer mixture of P(AAm-co-AN) was
469
successfully synthesized and was found to be more thermally stable than Gg. The synthesized
470
hydrogel polymer exhibited a maximum swelling capacity of 921% in neutral medium at 60
471
°C after 16 h. The Gg-cl-P(AAm-co-AN) hydrogel polymer also absorbed saline water from
472
different saline water-petroleum fraction emulsions, and the maximum absorption of saline
473
water was observed in the petroleum ether emulsion. Furthermore, the Gg-cl-P(AAm-co-AN)
474
hydrogel polymer was found to exhibit very good flocculation characteristics, with the
475
maximum flocculation efficiency observed with a 15 mg·l-1 polymer dose in acidic medium
476
at 40 °C. In soil composite, the Gg-cl-P(AAm-co-AN) hydrogel polymer was found to
477
degrade 89.47% within 60 days, and changes in surface morphology and chemical
478
composition were observed by SEM and FTIR. Moreover, The Gg-cl-P(AAm-AN) hydrogel
479
polymer successfully removed more than 98% of cationic dyes from aqueous solution. This
480
feasibility study confirmed that the hydrogel polymer synthesized here could be a good
481
adsorbent for the removal of cationic dyes from industrial waste water.
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Conclusions
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483
Acknowledgments
484
The authors are grateful to the National Research Foundation, South Africa for awarding a
485
postdoctoral research fellowship to Dr. Hemant Mittal. The authors (HM, AM, SSR) would
486
also like to thank the Department of Science and Technology and the Council for Scientific
487
and Industrial Research, South Africa for their financial support.
488
Page 20 of 37
Supplementary information
490
The supplementary document reports the TGA/DTA/DTG of the Gg-cl-P(AAM-co-AN)
491
hydrogel polymer and presents a comparison of the percentage thermal decomposition of the
492
Gg and Gg-cl-P(AAm-co-AN) hydrogel polymers.
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493
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and
flocculation
characteristics
of
ip t
Synthesis
cr
(2008).
us
H.Q.
d
M
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&Xie,
Ac ce pt e
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D.,
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638
639
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641
Captions to the Figures and Tables
642
Table 1:
emulsions. Table 2:
to the soil composting method. Fig. 1(a-c):
Effects of (a) time, (b) temperature and (c) pH on the Ps of the Gg-cl-P(AAm-
an
Fig. 2(a-c):
us
an image of a flower pot containing soil compost for biodegradation studies.
647
648
(a-b) Images of the sludge treatment plant at the NIT, Jalandhar, India and (c)
cr
645
646
Percentage weight loss of the Gg-cl-P(AAm-co-AN) hydrogel polymer subjected
ip t
643
644
Percentage removal of saline from different petroleum fraction-saline
co-AN) hydrogel polymer.
649
Effects of (a) time, (b) temperature, (c) pH on turbidity.
Fig. 3(a-c):
651
Figs. 4(a-d): FTIR spectra of (a) the Gg-cl-P(AAm-co-AN) hydrogel polymer, (b) the Gg-cl-
d
P(AAm-co-AN) hydrogel polymer in stage I of biodegradation (c) the Gg-cl-
Ac ce pt e
652
P(AAm-co-AN) hydrogel polymer in stage II of biodegradation, (d) the Gg-cl-
653
P(AAm-co-AN) hydrogel polymer in stage III of biodegradation.
654
655
Figs. 5(a-d): SEM images of (a) the Gg-cl-P(AAm-co-AN) hydrogel polymer, (b) the Gg-clP(AAm-co-AN) hydrogel polymer in stage I of biodegradation (c) the Gg-cl-
656
P(AAm-co-AN) hydrogel polymer in stage II of biodegradation, (d) the Gg-cl-
657
P(AAm-co-AN) hydrogel polymer in stage III of biodegradation.
658
659 660
M
650
Fig. 6:
Adsorption of methylene blue and rhodamine B using the Gg-cl-P(AAm-co-AN) hydrogel polymer.
661
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672
Highlights •
Hydrogel polymer of Gum ghatti with acrylamide-co-acrylonitrile was synthesized
674
•
Hydrogel polymer degraded 89.47% after 60 days using soil composting method
675
•
Hydrogel polymer showed pH and temperature responsive swelling studies
676
•
Hydrogel polymer showed maximum flocculation efficiency in acidic medium
677
•
Hydrogel polymer removed 98% of MB and RhB from aqueous solution
cr
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ip t
673
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Percentage removal of saline from different petroleum fraction-saline emulsions
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Table 1:
cr
ip t
Table(s)
Sample
Percentage saline removal Petrol-saline
Pet. ether-saline
% removal
±SD
±SE
% removal
±SD
±SE %removal ±SD
±SE
% removal
±SD
±SE
21.0
1.27
0.73
18.7
0.51
0.30
0.30
25.5
0.87
0.50
M
Gg-cl-P(AAm-co-AN)
Diesel-saline
an
Kerosene-saline
23.5
0.52
Ac c
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d
whereas, no. of replications = 03; amount of each emulsion taken= 100ml (1:1 v/v); weight of superabsorbent taken for each emulsion= 1.0g
Page 36 of 37
cr
ip t
Table(s)
Table 2: Percentage weight loss of Gg-cl-P(AAm-co-AN) hydrogel polymer using soil composting method Percentage weight loss at different time intervals (Days) 10
15
20
11.53
16.25
21.95
33.96
25
30
35
40
45
50
55
60
37.50
41.86
48.93
57.62
65.37
73.05
77.14
89.47
Ac c
ep te
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M
an
Gg-cl-P(AAm-co-AN)
5
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Sample Code
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